a. Field of the Invention
The present invention concerns the control of an imaging device. More particularly, the present invention concerns controlling the stepping of an x-ray angiographic imaging device to maximize the diagnostic and/or therapeutic usefulness of images acquired by the x-ray angiographic imaging device, while minimizing contrast medium and x-ray dosages to the patient.
b. Related Art
X-ray imaging devices have been important tools for medical diagnosis and intervention. For example, x-rays have been used to observe the blood vessels of patients. Typically, a radiopaque (i.e., opaque to radiation) substance (also referred to as "contrast media" or "bolus") is injected into a blood vessel under consideration. The blood vessel can be imaged based on x-rays of the blood vessel containing the radiopaque substance. This practice is known as angiography.
FIG. 1a is a known angiographic system 100 for studying the blood vessels in the left leg 104 and the right leg 106 of a patient 102 laying on table 160. Contrast media is injected into a blood vessel of the patient 102 with the contrast media injection means 150. The blood vessels, through which the contrast media will flow, are irradiated with x-rays from x-ray generator 111 and an image of an area of the patient 102 is captured by x-ray detector 110. The image is provided to visual output unit 120. As the blood flows down the left and right legs 104 and 106, respectively, of the patient 102, the contrast media will be carried down through the blood vessels of the left and right legs 104 and 106, respectively.
Unfortunately, the contrast media has some adverse side effects. First, patients often experience discomfort, typically a burning sensation, when and after the contrast media is injected. The severity and length of such discomfort will increase as the amount of contrast media injected increases. Second, the contrast media may adversely affect a patient's kidneys. The extent of the affect of the contrast media on the patient's kidneys will depend on the patient's renal condition and on the amount of contrast media used. Accordingly, the amount of contrast media used should be minimized--only being used to the extent needed to provide diagnostically useful images of a patient's blood vessels.
Since the amount of contrast media is limited, as the contrast media passes through a given point of a blood vessel, the contrast of a corresponding image will increase (known as "wash-in" period), reach a period of maximum contrast (known as a "plateau" period), and then decrease (known as "wash-out" period). Thus, the imaging device, which includes x-ray generator 111 and x-ray detector 110, should image a section of the legs 104/106 of the patient 102 when the contrast media within that section of the legs 104/106 reaches the period of maximum contrast. As the contrast media flows down through the legs 104/106 of the patient 102, the imaging device 110/111 must be moved in the direction Y, indicated by arrow 135, by means of an imaging device stepper 130. The x-ray generator 111 may be mechanically coupled, or controlled in unison, with the x-ray detector 110 such that both move together. The imaging device stepper 130 is controlled by step controller 140 which is operated by a medical professional (not shown). The medical professional (also referred to as "the operator" or "the angiographer") relies on the images provided by the visual output device 120 in judging when to command the image device stepper 130 to move the imaging device to a next position (also referred to as "gantry station(s)"). Since the imaging device stepper 130 cannot instantaneously move the imaging device 110/111 to the next station, a stepping time lag must be considered.
Unfortunately, due to limitations in human vision, reaction time, and judgment, the angiographer may command the imaging device stepper 130 to step the imaging device 110/111 too early or too late. If the imaging device 110/111 is stepped too early, the images acquired by the imaging device 110/111 will have been, and will be, acquired when the contrast fluid is in a wash-in period, before maximum contrast (or opacity) occurs. If, on the other hand, the imaging device 110/111 is stepped too late, the images acquired by the imaging device will be acquired after the contrast medium has reached maximum opacity--namely, either in "wash-out" period or a "plateau" period. If the image is acquired when the contrast fluid is in a "wash-out" period, after maximum contrast (or opacity) has occurred, the diagnostic usefulness of the acquired image will decrease. In addition, the patient 102 may be exposed to unnecessary extra x-ray dosage since frames acquired after the contrast medium has reached maximum opacity carry redundant information.
One way of maximizing the diagnostic usefulness of images acquired by the imaging device 110/111 would be to increase the amount of contrast media injected into a patient thereby increasing the (plateau) period of useful contrast between the wash-in and wash-out periods. By increasing the (plateau) period of useful contrast between the wash-in and wash-out periods, a greater margin of error in the stepping times is afforded. Unfortunately, however, increasing the amount of contrast media increases patient discomfort and increases the chances of adverse kidney complications as discussed above.
Another way of maximizing the diagnostic usefulness of the images acquired is to provide a number of imaging devices, or one large imaging device, that cover the entire field-of-view, each of which constantly acquire images so that stepping is not required. Unfortunately, such a scheme would dramatically increase (a) the patient's exposure to radiation and (b) the cost of the system.
Still another way of maximizing the diagnostic usefulness of the images acquired by the imaging device 110 is to extensively train medical personnel controlling the stepping of the imaging device 110/111 so that their judgment is improved. Unfortunately, angiographic imaging systems 100 are relatively expensive and thus, most medical facilities purchase only enough to meet their needs. Allocating training time decreases system throughput and, consequently, increases operating costs. Thus, needed training is difficult and costly to obtain.
Thus, a system which maximizes the diagnostic usefulness of angiographic images, using a judicious amount of contrast media, is needed. Such a system should permit off-line planning so that (a) an optimum volume of contrast media can be determined, (b) an optimum injection rate and/or pattern can be planned, (c) an optimum x-ray dose to which the patient will be exposed can be planned, and/or (d) optimum stepping times can be determined. Further, a system for training medical personnel to enhance their judgment as to stepping times is needed.